1. Field of the Invention
This invention relates to an automatic lens meter for measuring a spherical power, a cylinder power, an angle of cylinder axis and a power of prism of an ophthalmic lens.
2. Description of the Prior Art
Description will start with a principle of a telescopic lens meter and a projection-type lens meter, with reference to FIG. 1. Lenses 102 and 104 are disposed on the same optical axis at a spacing from each other. A lens stop 103 for holding thereon a lens 106 being inspected is placed between the lenses 102 and 104. A target 101 is placed at the left of the lens 102 in FIG. 1, and an image forming plane 105 is set on the second focal point of the lens 104. The flux of light from the target 101 becomes parallel between the lenses 102 and 104, and then the light transmitted through the lens 104 is focused onto the image forming plane 105 to form an image of the target 101, as seen in FIG. 1(a).
When the lens 106 being inspected is placed on the lens stop 103, as shown in FIG. 1(b), then the flux of light can no longer be parallel between the lenses 106 and 104, and hence no image is formed on the image forming plane 105. In order to form an image of the target 101 clearly on the image forming plane 105, the target 101 must be shifted along the optical axis. The length of shift of the target 101 has a linear relationship with the spherical power of the lens 106 being inspected. The spherical power of the lens being inspected is obtained by reading the length of shift of the target 101.
In the telescopic lens meter, a reticle is placed on the image forming plane 105; an image of the target 101 is focused onto the reticle, and the image thus formed on the plane is observed by a magnifier. In the projection-type lens meter, a diffused plate is placed on the image forming plane 105, and an image of the target is observed through the diffused plate.
In the telescopic lens meter, since the image of the target 101 formed on the reticle is observed in the form of an aerial image by means of the magnifier, the improper adjustment of the visibility leads to reading error. Furthermore, if a lens meter operator should have astigmatism in his eyes, it would be observed as if a spherical lens has a cylinder power. This is due to the fact that the optical system of the lens meter is joined coherently to the optical system of the operator's eyes, and the local performance of the optical system of the operator's eyes, which permits transmission of the flux of light emitted from the lens meter, largely influences how the image of the target 101 is seen.
Regarding the projection-type lens meter, since the flux of light from the target 101 is projected on to the diffused plate; the image of the target 101 is observed over the entire zone of the pupils of the operator's eyes, and hence the local performance of the optical system of one's eyes has only a little influence on how the image is observed. Yet, there remains an influence of abberrations occurring over the entire zone of the pupils of the eyes.
Description is proceeded on the measurement of a ophthalmic lens having a cylinder power by means of the telescopic lens meter or the projection-type lens meter. In this case, either type lens meter can produce the same results. To the effect that a refractive power for a principal meridian is obtained, the target 101 is shifted along the optical axis, and at the same time, the target 101 is turned about the optical axis, in a kind of lens meter. The shift and rotation of the target 101 on the optical axis are repeated, so that, based on the length of shift of the target 101 and an angle of rotation thereof, the refractive power for the principal meridian and directions thereof are respectively obtained. In a lens meter in which the turning of the target 101 is not permitted, the reticle is rotated to coincide the reticle line with a direction of flux of an image of the target 101, and an angle of rotation of the reticle is read by means of a graduation panel attached to the reticle, whereby an operator can know a direction of the principal meridian.
In order to obtain a refractive power for another principal meridian rectangular to the aforesaid principal meridian, the target 101 again shifted to a proper point. Based on the two refractive powers and directions of two principal meridians thus obtained, the spherical power, cylinder power and an angle of cylinder axis are determined according to a known calculation formula.
The telescopic-type lens meter and the projection-type lens meter all use the eyesight of an operator, thus resulting in a difference in measurement among individuals. Furthermore, such lens meters are hard to operate, and hence require skill.
To cope with the above drawbacks, an automatic lens meter comes to practical use. One such lens meter is disclosed in U.S. Pat. No. 3,870,415. In the automatic lens meter disclosed therein, laser beam have been used for a light source, so that the laser beam are caused to circularly scan on a lens being inspected by means of a little deviated prism rotation at a given rate. The beam refracted by the lens being inspected scan on a ruling rotating at a predetermined speed, and the frequency of the light chopped by the ruling is sensed and counted by a sensor disposed at the rear of the ruling, whereby the spherical power inspected, the cylinder power and the agnel of cylinder axis, of the lens being inspected, are calcualted.
Other automatic lens meters are disclosed in Laid-open Japanese Patent Publications No. 50-14757 and 54-14758. These automatic lens meters are such that the flux of light from four light sources separated by prisms are shielded by a shading means having a special pattern thereon and rotating at a predetermined rate, and the light thus shielded is caused to enter a lens being inspected. Among the flux of light rays refracted from the lens being inspected, the light rays parallel to the optical axis are allowed to transmit through a means 101 selectively transmitting the light rays, and a shading time lag along the flux of light from respective light sources is sensed by a photosensor disposed at the rear of the means for selectively transmitting the light rays, whereby the spherical power, the cylinder power and the angle of cylinder axis are calculated.
Those automatic lens metters, however, are attended with the following drawbacks.
The firstly mentioned automatic lens meter requires two rotary mechanisms for providing a predetermined rotation, resulting in a complicated construction, and, hence, assembly, adjustment and inspection are time-consuming. Furthermore, a measuring accuracy is impaired, particularly in the case where the ruling fluctuates in the radial direction. In order to provide an improved measuring accuracy, special parts such as laser light sources must be employed, and/or permissible error of respective components must be small. Thus, an expensive automatic lens meter is bound to result.
In the second mentioned automatic lens meter, the shading means is a rotary disc consisting of transparent portions and opaque portions, each of which has a peculiarly curved pattern, and the polar coordinates of a point at which the light ray crosses the rotary disc is obtained from an angle of rotation of the rotary disc. For this reason, the peculiar curved patterns of the rotary disc must be highly precise, in order to achieve the practical accuracy. In order to maintain the rectangular coordinate uniformly accurate over the entire zone of the flux of light being shaded, because a reading must be converted into rectangular coordinates, precision of the curved pattern must be enhanced toward the center of rotation of the rotary disc. Furthermore, the rotary disc must be disposed vertically highly accurately with respect to the optical axis as well as a rotary shaft of a motor. The rotary shaft of the motor must exactly meet the origin of the polar coordinates of the rotary disc. However, a fluctuation in the radial direction of the rotary means occasionally occurs, with the failure to match the origin of the polar coordinates with the rotary shaft of the motor, and the origin of the polar coordinates is rotated, resulting in inaccurate measurement.
The second automatic lens meter is different from the first lens meter in the points that the white light source is employed and only a single rotary mechanism is incorporated. So far as these points are concerned, the manufacturing cost would be reduced to some extent. However, use of the shading means of a peculiar configuration increases a cost to an extent more than that.